Nativism is essentially the belief that the people who were born in a country should be favored rather than immigrants. This idea flared up in some of the American people after World War I because of patriotism, isolationism, and also the Red Scare.
After World War I, many people became isolationists, even some people in Congress. This meant that they did not want to become involved with foreign nations because they feared joining another war. Isolationism, along with the newfound patriotism that Americans found after the war, caused many people to not want immigrants in the country and wanted themselves to be favored.
The Red Scare also contributed to the flare-up of nativism after World War I. The First Red Scare happened after the Bolshevik Revolution and was a time where many Americans feared communism would spread to the United States and around the world. Some people even believed there were spies in the government trying to spread communism. This caused a lot of people to not want immigrants to enter the country, as they believed they were communists.